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The Endogenous Circadian Pacemaker Imparts a Scale-Invariant Pattern of Heart Rate Fluctuations across Time Scales Spanning Minutes to 24 Hours

Division of Sleep Medicine, Brigham and Women's Hospital, Boston, Massachusetts, khu{at}bidmc.harvard.edu

Frank A. J. L. Scheer

Division of Sleep Medicine, Brigham and Women's Hospital, Boston, Massachusetts, Department of Hypothalamic Integration Mechanisms, Netherlands Institute for Brain Research, Amsterdam, The Netherlands

Ruud M. Buijs

Department of Hypothalamic Integration Mechanisms, Netherlands Institute for Brain Research, Amsterdam, The Netherlands, Department Physiology, Instituto Biomedicas, Universidad Nacional Autónoma de México, Mexico City, Mexico

Steven A. Shea

Division of Sleep Medicine, Brigham and Women's Hospital, Boston, Massachusetts, Steven_shea{at}hms.harvard.edu

Heartbeat fluctuations in mammals display a robust temporal structure characterized by scale-invariant/fractal patterns. These scale-invariant patterns likely confer physiological advantage because they change with cardiovascular disease and these changes are associated with reduced survival. Models of physical systems imply that to produce scale-invariant patterns, factors influencing the system at different time scales must be coupled via a network of feedback interactions. A similar cardiac control network is hypothesized to be responsible for the scale-invariant pattern in heartbeat dynamics, although the essential network components have not been determined. Here is shown that scale-invariant cardiac control occurs across time scales from minutes to ~24 h, and that lesioning the mammalian circadian pacemaker (suprachiasmatic nucleus; SCN) completely abolishes the scale-invariant pattern at time scales >~4 h. At time scales <~4 h, the scale invariance persisted following SCN lesion but with a different pattern. These results indicate previously unrecognized multiscale influences of the SCN on heart rate fluctuations that cannot be explained by a simple pacemaker of 24-h rhythmicity. The conclusion is that the SCN serves as a major node in the cardiac control network and imparts scale-invariant cardiac control across a wide range of time scales with strongest effects between ~4 and 24 h. These results demonstrate that experimental manipulations (e.g., SCN lesion) can be used to begin to model and understand the origin of scale-invariant behavior in a neurophysiological system.

Key Words: cardiac control • scale-invariant patterns • the suprachiasmatic nucleus (SCN) • SCN lesion • network • feedback interactions

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				K. Hu, F. A.J.L. Scheer, R. M. Buijs, and S. A. Shea The circadian pacemaker generates similar circadian rhythms in the fractal structure of heart rate in humans and rats Cardiovasc Res, June 27, 2008; (2008) cvn150v2. [Abstract] [Full Text] [PDF]